The present invention relates to motor vehicle seats with a seat portion and a back rest whose upright position can be adjusted using a tilt adjustment device. This invention has particular application in connection with the back seats of automobiles, but may be used with other motor vehicle seats if desired.
U.K. patent application No. 2,300,352 discloses a motor vehicle seat in which the back rest pivots around a horizontal pivot axis at the bottom end of the back rest. The angle of pivot is determined by a clutch device that makes it possible to lock the back rest in two different positions relative to the automobile car body. It is not possible to adjust the back rest at other positions.
An adjustment device in which the angle of the back rest can be continuously adjusted, relative to the seat portion, is disclosed in U.S. Pat. No. 4,668,013. Two hinge portions, one of which is attached to the seat portion and the other of which is attached to the back rest, are connected through a planetary gear train. More particularly, one hinge portion has a pair of coaxial sun gears, i.e., gears with internally facing gear teeth, of differing diameters. The other hinge portion has a corresponding pair of planet gears, i.e., gears with externally facing gear teeth. Each planet gear has a slightly smaller outer diameter, and one less gear tooth, than the corresponding sun gear. The planet gears are eccentrically mounted relative to the sun gears on a common crank, so that the upon rotation of the crank, the planet gears move in an orbital path relative to the sun gears. An adjustment knob is provided for rotating the crank. Due to the fact that each planet gear has one less gear tooth than the corresponding sun gear, the orbital motion of the planet gears, caused by rotating the knob, causes a small amount of rotation between the two hinge portions, thereby changing the tilt angle of the back rest.